The present invention relates to a method of generating S wave in ground formation for the purpose of a geological survey and the like.
As well known to those skilled in the art, the elastic waves propagated through the ground consist of three types, namely longitudinal wave (P wave), shear wave (S wave) and surface wave. Thus, the term "S wave" is used throughout the specification to mean the shear wave of the elastic waves.
A method has been mainly used in the field of seismic technology and gelogical survey, which includes inserting a wave source in a bore hole, and actuating an operating mechanism in the wave source to impart a exciting force to the wall of the bore hole in the direction perpendicular to the axis of the bore hole, thereby to generate S wave having characteristics of predominant radiation in the direction perpendicular to the axis of the exciting force. This method is actually accomplished by (1) directly striking the wall of the bore hole by means of a hammer, or (2) by generating an explosive fluid pressure to impart an impact to the wall of the bore hole.
However, the above-mentioned method (1) relying upon a direct mechanical impact inconveniently requires a special mechanism for securing the wave source to the wall of the hole. In addition, the handling and operation of the wave source is extremely difficult especially when the wave source is inserted to a large depth. Also, the crumbling of the hole wall is likely to occur when the wave source is operated in the hole.
The second method (2) is to generate an explosive fluid pressure in a rigid casing having an opened window by means of powder explosion, abrupt release of gas pressure or rapid gasification of liquid by high-voltage discharge and to erupt the pressure through the aforementioned window to a portion of the hole wall facing the window. Thus, the nature of the impact to the portion of the hole wall is fundamentally equivalent to that of the first method; rather, the second method involves additional problems. They are that
(a) the volume change of the water surrounding the wave source is more likely to induce the generation of the obstructive waves, namely P wave and tube wave; PA1 (b) the use of high-voltage is apt to generate undesirable induction noises; and PA1 (c) the electric discharge and release of compressed gas at a large depth require complex mechanisms and sophisticated control device. Due to these problems the application of the second method is inevitably limited.
In these methods, it is presupposed that the wave length of the generated wave is sufficiently longer than the hole diameter, and hence the wave detected by receivers is the direct S wave. In fundamental aspects, these points are also available in the present invention.
On the other hand, there is also an S wave logging method belonging to other category, completely different from that of the above methods, in which the wave length of the generated wave is shorter than or comparable to the hole diameter. In this method, a P wave is originally emitted to surrounding water from a source having characteristics of omni-directional radiation, and then at the hole wall near the source, this wave is converted to S wave propagating in solid formation along the hole wall. This S wave is then refracted to water in the hole as P wave, which is detected by receivers. In essential meaning, a logging of this type is "a conventional sonic log" modified to detect later phases, where the refracted S wave (P.fwdarw.S.fwdarw.P) also appears as by-production of P wave in favourable conditions. This method has such a restriction in application that characteristics of the refracted S wave are disturbed by near wall condition of the formation including drilling effects and it cannot be applied to formations of lower S wave velocity.